This invention relates to a method and apparatus for treating exhaust gases of an internal combustion engine to reduce the quantity of pollutants expelled into the atmosphere, particularly during engine warm-up.
Automotive vehicles have been equipped with catalytic converters for many years. The catalytic converter performs well to remove pollutants from the vehicle engine exhaust gases during most operating conditions. However, catalytic converters require a significant amount of heat energy to ignite and begin catalytic action. Accordingly, large quantities of pollutants are released into the atmosphere in the first few minutes after engine start-up due to the delay in heating the catalytic converter to the ignition or reaction temperature. It has been proposed that gases be preheated by dissipation heaters to shorten this delay, however, the drain on the battery caused by this heater is so great that a separate battery may be required. The purpose of the invention is to reduce, as much as possible, the time necessary for a cold catalytic converter to light off (ignite) to reduce the quantity of pollutants expelled into the atmosphere during the warm-up period. This is accomplished through the action of a catalytic preheater and bypass, with or without an electrical preheater. The catalytic preheater is designed and utilized in such a manner that it extracts the maximum amount of energy from catalytic oxidation of the exhaust stream, warming the gasses which feed into the converter so that the light off behavior is improved.
A conventional catalytic converter will not rapidly ignite unless inlet gases from the exhaust manifold of the automobile engine are at 700xc2x0 K. or above. Unfortunately, the gases from an initially cold manifold begin at approximately 300xc2x0 K., and ramp up to a steady temperature of 600xc2x0 K. As a consequence, the light off behavior of the standard catalytic converter is poor. One technique which has been proposed to improve this performance is to preheat the gases entering the converter by using an electrical dissipation heater. However, the amount of electrical energy required to heat the cold exhaust gases to the desired temperature of 700xc2x0 K. is so large that a separate battery may be required, particularly in cold climates. The proposed invention addresses this problem by using the heat content of the exhaust gases themselves to aid in the preheating process. The catalytic preheater can be used with or without an electrical preheater, although the light off performance will be better with some electrical preheating.
According to the present invention, a catalytic heater or igniter is placed in the engine exhaust pipe upstream of the catalytic converter. The igniter contains a catalyst that heats a portion of the exhaust gases, while the remainder of the exhaust gases are bypassed around the igniter. The igniter raises the temperature of gases 300xc2x0 K. through oxidation of residual CO and H2 pollutant in exhaust, so that, when combined with the gases bypassed around the igniter, the average temperature of the exhaust gas stream is raised from about 600xc2x0 K. to 700xc2x0 K., which is sufficient to ignite the catalytic converter at the leading edge thereof. Calculations show that the total ignition time is reduced from several minutes to approximately thirteen seconds for warm manifold exhaust gases, thereby reducing the release into the environment of carbon monoxide by a factor of four and nitrous oxides and hydrocarbons by a factor of ten during the cold converter start up period.
According to the present invention, a catalytic preheater or igniter is placed in the engine exhaust system upstream of the catalytic converter. The igniter is capable of raising the temperature of the exhaust gases from the initial temperature of 600xc2x0 K., which is the temperature of the exhaust gases at the engine manifold, to an adiabatic temperature of about 900xc2x0 K. Since an ordinary catalytic converter will rapidly ignite at its leading edge if the temperature of the exhaust gas is raised to 700xc2x0 K., only about one third of the exhaust gases need to pass through the catalytic preheater or igniter. This reduced flow allows the preheater to be much shorter than the catalytic converter. The rest of the exhaust gas is bypassed around the catalytic preheater, and the gases are recombined at the inlet of the catalytic converter. Accordingly, the temperature of the combined gases is raised from 600xc2x0 K. to 700xc2x0 K.
If an electrical preheater is used, the amount of energy available to the electrical preheater depends on the battery type and operating conditions such as temperature. There will exist some maximum allowable draw on the battery, which will likely be a function of time after engine ignition. It is proposed to draw close to this maximum amount to warm the gases as much as possible. In order to promote rapid ignition in the catalytic preheater, however, sufficient gases are bypassed so that the electrical heater (with the allowable draw on the battery) can warm the gases to approximately 700xc2x0 K. At this temperature the catalytic preheater rapidly ignites in a few seconds. The fraction of the gases passing through the electrical heater and catalytic preheater is then increased as much as possible without 1) the ignition in the preheater being extinguished or 2) the pressure drop becoming too large. The gases passing through the electrical heater and catalytic preheater are then mixed with the bypassed gases and enter the converter. The warmed gases then rapidly ignite the catalytic converter. When the converter reaches the desired operating temperature, the electrical preheater is shut off, and all gases are bypassed around the catalytic preheater and electrical preheater. This serves to preserve the life of the oxidation catalyst in the catalytic preheater. The optimum bypass and catalytic preheater of the embodiment are similar to those required without an electrical preheater, except that the controller adjusts the amount of gas flowing through the bypass and through the preheater. This can be accomplished by sensing the temperature of the gas stream leaving the electrical preheater: from a measured temperature rise and known energy input into the electrical preheater it is possible to determine the gas mass flow rate and adjust it accordingly.
Accordingly, the catalytic preheater may use a much smaller pore size, thereby improving the coupling between the gas temperature and the solid temperature, that is, the temperature of the catalyst is raised quite quickly to the temperature of the incoming gas without significantly increasing the pressure drop through the igniter and without requiring use of a high catalyst loading, which would substantially increase the cost. The normal catalytic converter may be made to ignite more quickly by increasing catalyst loading, but if all of the gas must pass through the catalytic converter, increasing the catalyst loading substantially increases the size and cost of the catalytic converter. Furthermore, since only one third of the exhaust gas passes through the catalytic preheater, the length of the preheater may be substantially reduced. The combination of all these effects results in a catalytic preheater which ignites after about 7 seconds for exhaust gases at 600xc2x0 K., such that the catalytic preheater raises the temperature of the combined exhaust gases at the inlet of the catalytic converter to the required 700xc2x0 K. The temperature of the combined exhaust gases at the inlet of the catalytic converter reaches this temperature about 6 seconds after the catalytic preheater ignites. Convection of the hot exhaust gases through the catalytic converter brings the temperature of the catalyst within the converter to a uniform reaction temperature in about 15 additional seconds. Accordingly, the catalytic converter is ignited in about 28 seconds. Accordingly, the requirements of the catalytic preheater are (1) a pore size as small as possible (down to about 300 xcexcm radius, half of a conventional monolith radius) without significantly increasing the solid support heat capacity, (2) a catalyst loading sufficient for an ignition time of about 2.4 to 6 seconds at 600xc2x0 K. inlet gas temperature (which is achievable with commercially available loadings), and (3) a length greater than the dispersion free ignition length (about 1.5 cm for typical loading and gas flow rate conditions). Such a design uses about half of the catalyst in an ordinary catalytic converter, and has an additional pressure drop less than half of a typical converter. If the bypass were not used, the catalyst required in the catalytic preheater increase by a factor of three and the pressure drop across the preheater would increase by a factor of nine.
Accordingly, by bypassing most of the exhaust gases around the preheater, an economical preheater may be provided which does not detrimentally affect the overall system. Because the catalytic preheater only warms the exhaust gases, it is not necessary to employ the complex three way catalyst used in the regular catalytic converter. Such oxidation catalysts are readily available. Accordingly, the present invention dramatically decreases ignition time of a catalytic converter, thereby substantially reducing the quantity of pollution expelled by motor vehicles during the converter warm up and ignition. This is accomplished with minimal additional cost, and substantially no detrimental affect on overall exhaust gas system performance.